1. Field of the Invention
The present disclosure relates generally to power transfer systems for controlling the distribution of drive torque between the front and rear drivelines of a four-wheel drive vehicle and/or the left and right wheels of an axle assembly. More particularly, the present disclosure is directed to a power transmission device for use in motor vehicle driveline applications having a torque transfer mechanism equipped with a power-operated clutch actuator that is operable for controlling actuation of a multi-plate friction clutch assembly.
2. Related Art
This section provides background information related to the present disclosure which is not necessarily prior art.
In view of increased demand for four-wheel drive and all-wheel drive vehicles, a plethora of power transfer systems have been developed for incorporation into vehicular driveline applications for transferring drive torque to the wheels. In many vehicles, a power transmission device is operably installed between the primary and secondary drivelines. Such power transmission devices are typically equipped with a torque transfer mechanism which is operable for selectively and/or automatically transferring drive torque from the primary driveline to the secondary driveline to establish a four-wheel drive or all-wheel drive mode of operation.
A modern trend in four-wheel drive motor and all-wheel drive vehicles is to equip the power transmission device with a transfer clutch and an electronically-controlled traction control system. The transfer clutch is typically operable for automatically directing drive torque to the secondary wheels, without any input or action on the part of the vehicle operator, when traction is lost at the primary wheels for establishing an “on-demand” four-wheel drive mode. Typically, the transfer clutch includes a multi-plate clutch assembly that is installed between the primary and secondary drivelines and a clutch actuator for generating a clutch engagement force that is applied to the multi-plate clutch assembly. The clutch actuator typically includes a power-operated device that is actuated in response to electric control signals sent from an electronic controller unit (ECU). Variable control of the electric control signal is frequently based on changes in one or more of the the current operating characteristics of the vehicle (i.e., vehicle speed, interaxle speed difference, acceleration, steering angle, etc.) as detected by various sensors. Thus, such “on-demand” power transmission devices can utilize adaptive control schemes for automatically controlling torque distribution during all types of driving and road conditions.
A large number of on-demand power transmission systems have been developed which utilize an electrically-controlled clutch actuator for regulating the amount of drive torque transferred through the clutch assembly to the secondary driveline as a function of the value of the electrical control signal applied thereto. In some applications, the transfer clutch employs an electromagnetic clutch as the power-operated clutch actuator. For example, U.S. Pat. No. 5,407,024 discloses an electromagnetic coil that is incrementally activated to control movement of a ball-ramp drive assembly for applying a clutch engagement force on the multi-plate clutch assembly. Likewise, Japanese Laid-open Patent Application No. 62-18117 discloses a transfer clutch equipped with an electromagnetic clutch actuator for directly controlling actuation of the multi-plate clutch pack assembly.
As an alternative, the transfer clutch may employ an electric motor and a drive assembly as the power-operated clutch actuator. For example, U.S. Pat. No. 5,323,871 discloses an on-demand transfer case having a transfer clutch equipped with an electric motor that controls rotation of a sector plate which, in turn, controls pivotal movement of a lever arm for applying the clutch engagement force to the multi-plate clutch assembly. Moreover, Japanese Laid-open Patent Application No. 63-66927 discloses a transfer clutch which uses an electric motor to rotate one cam plate of a ball-ramp operator for engaging the multi-plate clutch assembly. U.S. Pat. No. 4,895,236 discloses a transfer case equipped with a transfer clutch having an electric motor driving a reduction gearset for controlling movement of a ball screw operator which, in turn, applies the clutch engagement force to the clutch pack. Additionally, U.S. Pat. No. 5,423,235 discloses a transfer case equipped with an electric motor driven gearset arranged to rotate a first cam member of a hall-ramp operator for causing axial translation of a second cam member configured to apply the clutch engagement force on the clutch pack. Finally, U.S. Pat. Nos. 7,527,133 and 8,479,904 disclose transfer cases equipped with an electric motor driven gearset arranged to both rotate and axially translate a single cam member of a ball-ramp operator.
While many on-demand clutch control systems similar to those described above are currently used in four-wheel drive and all-wheel drive vehicles, a need exists to advance the technology and address recognized system limitations. For example, the size and weight of the friction clutch components and the electrical power and actuation time requirements for the clutch actuator that are needed to provide the large clutch engagement loads may make such system cost prohibitive in some motor vehicle applications. In an effort to address these concerns, new technologies are being considered for use in power-operated clutch actuator applications.